[SC2116] exception: $(echo ...) used to compose regular expression patterns

[64kramsystem]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/bash

[[ 123 =~ $(echo '^123\b') ]] && echo match

Here's what shellcheck currently says:

Line 3:
[[ 123 =~ $(echo '^123\b') ]] && echo match
          ^-- SC2116: Useless echo? Instead of 'cmd $(echo foo)', just use 'cmd foo'.

Here's what I wanted or expected to see:

No warnings.

Since non-trivial regular expressions (to be matched with the =~ operator) can't be written inline, one needs a workaround, typically $(echo <expression>), which ideally should not raise a warning.

I'm also a bit perplexed about Shellcheck recognizing the =~ operator as a command, which causes the warning to be raises (however, I'm not familiar with the internals).